1. Field of the Invention
This invention relates generally to deceleration control systems for vehicles, and more particularly concerns a brake energy balancing system for multiple brake units using multiple independent brake controllers to control deceleration by maintaining braking torque constant or within a desired torque range.
2. Description of Related Art
Anti-skid and automatic braking systems have been commonly provided on commercial aircraft to aid the deceleration of the aircraft upon landing. Modern anti-skid systems commonly optimize braking efficiency by adapting to runway conditions and other factors affecting braking to maximize deceleration, corresponding to the level of brake pressure selected by the pilot. Such brake units have been proposed for use in emergency braking systems for magnetic levitation (MAG-LEV) trains for emergency deceleration in the event of failure of the primary braking system. However, MAG-LEV trains commonly have articulated trucks with numerous sets of independent brake units, over which brake pressure can differ, so that the energy absorbed by different brakes during braking can vary. It has been found that during braking of vehicles with independent controllers and brake units, such as in a train, can suffer from an energy imbalance among the brake units, despite anti-skid and autobrake deceleration control.
The causes of divergence of brake pressure can be attributed to (a) differences in the rolling radii of train wheels, (b) differences in electronic components, (c) brake torque differences, and the like. Each of these can create a measured `error` in the deceleration. When imbalances in braking energy occur, brake units that absorb more energy can fail temporarily, or become permanently damaged. It would, therefore, be desirable that the braking energy absorbed by the brake units of the trucks and cars of a train be equalized as much as possible. Conventional brake control systems having anti-skid and autobrake functions have been found to be insufficient to achieve such brake energy equalization among multiple independent brake units.
One prior art method for avoiding brake pressure imbalances in aircraft brakes equalizes the pressure at all wheels in auto-brake mode, with brake torque limiting as an auxiliary control mechanism. However, the anti-skid and autobrake functions are controlled by measuring velocity using a speed sensor. Since deceleration is determined from wheel speed, the calculation of deceleration from one truck to the next can be off by a factor related to the tolerance within the system. Such a tolerance deviation in determination of deceleration can cause a brake energy imbalance in the braking effort among trucks linked in a chain. Without an energy balancing method, a major portion of the braking effort can thus be borne by few of the trucks. In an emergency braking situation, one car can push or pull against an adjacent car, causing unsafe jostling of the train cars, which can be particularly dangerous for high speed trains. It is, therefore, desirable that in order to equalize brake energy, torque value of each brake should be measured and controlled. Thus, it would be desirable to employ an auto-torque braking energy balancing system and method that would maintain deceleration torque to a constant value or range of values. The present invention addresses these needs.